It’s not about not being able to port it; it’s not the right tool for the job. ZFS is a file system designed for servers; the Apple Watch, Apple TV and the iPhone and iPad aren’t servers.

That doesn't mean it can't be adapted and tuned for mobile. The average smartwatch has more processing power than the average server did at the time the ZFS project was launched (2001).

+1 to that. Or at least some subset that speaks the ZFS-protocol if such a thing exists.